Methods and Systems for Controlling a Hybrid Heating System

ABSTRACT

In at least some embodiments, a hybrid heating system includes a heat pump and an auxiliary furnace. The system also includes a controller coupled to the heat pump and the auxiliary furnace. The controller, in response to receiving a heat request, selects either the heat pump or the auxiliary furnace based on an economic balance point algorithm.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

In a heat pump and refrigeration cycle, refrigerant alternately absorbsand rejects thermal energy as it circulates through the system and iscompressed, condensed, expanded, and evaporated. In particular, a liquidrefrigerant flows from a condenser, through an expansion device (e.g.,expansion valve) and into an evaporator. As the refrigerant flowsthrough the expansion device and evaporator, the pressure of therefrigerant decreases, the refrigerant phase changes into a gas, and therefrigerant absorbs thermal energy. From the evaporator, the gaseousrefrigerant proceeds to a compressor, and then back to the condenser. Asthe refrigerant flows through the compressor and condenser, the pressureof the refrigerant is increased, the refrigerant phase changes back intoa liquid, and the refrigerant gives up thermal energy. The process isimplemented to emit thermal energy into a space (e.g., to heat a house)or to remove thermal energy from a space (e.g., to air condition ahouse).

In a heating cycle, the efficiency of a heat pump system is reduced asthe outdoor temperature drops. In other words, for every heat pumpsystem, there is an outdoor temperature threshold (referred to herein as“the thermal balance point”) below which the heat pump system is nolonger effective. Accordingly, some heating, ventilation, and airconditioning (HVAC) systems implement a hybrid (or dual) fuel system forheating, which comprises a heat pump system and an auxiliary furnace.The auxiliary furnace may burn gas, oil, propane or other combustibles.With the auxiliary furnace, the hybrid fuel system is capable of heatingan indoor environment even if the outdoor temperature drops below thethermal balance point of the heat pump system.

SUMMARY OF THE DISCLOSURE

In at least some embodiments, a hybrid heating system includes a heatpump and an auxiliary furnace. The hybrid heating system also includes acontroller coupled to the heat pump and the auxiliary furnace. Thecontroller, in response to receiving a heat request, selects either theheat pump or the auxiliary furnace based on an economic balance pointalgorithm.

In at least some embodiments, a control system for a hybrid heatingsystem includes economic balance point logic configured to determine anoutdoor temperature threshold at which operating an auxiliary furnace isless expensive than operating a heat pump. The control system alsoincludes selection logic configured to select, in response to a heatrequest, either the auxiliary furnace or the heat pump based on theoutdoor temperature threshold.

In at least some embodiments, a method for controlling a hybrid heatingsystem includes determining, by a controller, an outdoor temperaturethreshold at which operating an auxiliary furnace is less expensive thanoperating a heat pump. The method also includes receiving, by thecontroller, a heat request. The method also includes selecting, by thecontroller, either the auxiliary furnace or the heat pump based on thedetermined outdoor temperature threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an HVAC system with hybrid heating in accordance withan embodiment of the disclosure;

FIG. 2 illustrates a control system configuration for the HVAC system ofFIG. 1 in accordance with an embodiment of the disclosure;

FIG. 3 illustrates a block diagram of a system in accordance with anembodiment of the disclosure;

FIGS. 4A-4J show windows of a user interface program for controllinghybrid heating in accordance with an embodiment of the disclosure; and

FIG. 5 shows a method in accordance with an embodiment of thedisclosure.

DETAILED DESCRIPTION

FIG. 1 illustrates an HVAC system 100 with hybrid heating in accordancewith an embodiment of the disclosure. In the HVAC system 100,refrigerant cycles through a heat pump comprising outdoor coil 102,compressor 106, indoor coil 122, and expansion valve 112. The arrows104, 108, 110 and 114 show the direction of flow for refrigerant in aheating cycle. For a cooling cycle, the direction of flow forrefrigerant in HVAC system 100 would be reversed.

In a heating cycle, the outdoor coil 102 causes refrigerant toevaporate. As the liquid refrigerant evaporates it pulls heat from theoutside air. The gaseous refrigerant flows (arrow 104) from the outdoorcoil 102 to compressor 106, where the gaseous refrigerant is compressedto produce a high-pressure, superheated refrigerant vapor. The vaporleaves compressor 106 and flows (arrow 108) to the indoor coil 122. Atthe indoor coil 122, air from fan (blower) 124 removes heat from thevapor (warming the indoor air) and, when enough heat is removed, thevapor condenses into a high-pressure liquid. This high-pressure liquidflows (arrow 110) from the indoor coil 122 to the expansion valve 112,which meters the flow (arrow 114) of the high-pressure liquid to theoutdoor coil 102. The heating cycle process described herein can berepeated as needed. For example, the heating cycle of HVAC system 100may be activated and/or maintained in response to a thermostat controlsignal.

As shown in FIG. 1, the indoor coil 122 and the fan 124 may becomponents of an air handler 120. The air handler 120 may also comprisean auxiliary furnace 126, which is selectively activated as part of ahybrid heating scheme as disclosed herein. Alternatively, the auxiliaryfurnace 126 may be separate from the air handler 120. In either case,the auxiliary furnace 126 may be selectively activated (e.g., instead ofthe heat pump components) based on an economic balance point algorithm.In operation, the economic balance point algorithm determines whenoperating the heat pump of HVAC system 100 is more expensive to run thanthe auxiliary furnace 126. In such case, the economic balance pointalgorithm causes the auxiliary furnace 126 to run instead of the heatpump. The economic balance point algorithm also may account for userinputs to adjust or override the determined economic balance point asdescribed herein.

FIG. 2 illustrates a control system configuration 200 for the HVACsystem 100 of FIG. 1 in accordance with an embodiment of the disclosure.The control system configuration 200 illustrates a hierarchical controlfor HVAC systems, including those with hybrid heating as disclosedherein. As shown, the thermostat 202 operates as the overall systemcontroller of configuration 200 and is configured to communicate with anindoor subsystem controller 222 of indoor subsystem 220 and an outdoorsubsystem controller 212 of outdoor subsystem 210. The indoor subsystem220 may comprise, for example, indoor heat pump components 224 (e.g.,indoor coil 122 and fan 124) and auxiliary furnace components 226 (e.g.,auxiliary furnace 126) such as those described for FIG. 1. Meanwhile,the outdoor subsystem 210 comprises outdoor heat pump components 214such as the compressor 106 and the outdoor coil 102 described forFIG. 1. In at least some embodiments, the indoor subsystem controller222 implements some or all of the economic balance point algorithmfeatures described herein.

FIG. 3 illustrates a block diagram of a system 300 in accordance with anembodiment of the disclosure. As shown, the system 300 comprises acontroller 310 coupled to a hybrid heating system 320 having a heat pump322 and an auxiliary furnace 324. In at least some embodiments, thecontroller 310 and the user interface 302 corresponds to the indoorsubsystem controller 222 of FIG. 2. In various embodiments, the userinterface 302 corresponds to an interface on a thermostat or othercontrol unit that enables user interaction to control operations of thehybrid heating system 320. Alternatively, the user interface 302 maycorrespond to a computer program or web portal accessible via a handheldcomputing device (e.g., a smart phone), a laptop and/or a desktopcomputer.

As shown, the controller 310 comprises economic balance point logic 312configured to select whether to operate the heat pump 322 or theauxiliary furnace 324 in response to a heat request. In accordance withat least some embodiments, the economic balance point logic 312 employscontrol parameters 314 to determine when operating heat pump 322 is moreexpensive than operating auxiliary furnace 324. Values for the controlparameters 314 may be based on previously stored default values and/ordynamic values received via a user interface 302 coupled to thecontroller 310. As an example, the control parameters 314 may correspondto an auxiliary furnace fuel cost parameter, a heat pump electricitycost parameter, a heat pump efficiency parameter, and an auxiliaryfurnace efficiency parameter. Using such control parameters 314, theeconomic balance point logic 312 determines an outdoor temperaturebalance point at which operating the heat pump 322 is more expensivethan operating the auxiliary furnace 324.

The outdoor temperature balance point may be determined before or aftera heat request is received. In either case, the economic balance pointlogic 312 may respond to a heat request by comparing a current outdoortemperature with the determined outdoor temperature balance point, andthen selecting either the heat pump or the auxiliary furnace based onthe comparison.

More specifically, in FIG. 3, the selection logic 316 coupled to theeconomic balance point logic 312 may receive a recommendation or controlsignal from the economic balance point logic 312. In response to acontrol signal from the economic balance point logic 312, the selectionlogic 316 asserts a control signal to activate either the heat pump 322or the auxiliary furnace 324. In accordance with at least someembodiments, the heat pump 322 and the auxiliary furnace 324 areindependently activated, but are not typically operated together.

The selection logic 316 is also configured to receive a manuallyselected control scheme for the hybrid heating system 320 from the userinterface 302. The manually selected control scheme may correspond toadjusting or overriding the determined outdoor temperature balance pointdiscussed previously. In other words, the user interface 302 enables auser to selectively disable and enable the economic balance pointalgorithm performed by the economic balance point logic 312.Additionally or alternatively, the user interface 302 enables a user tomanually set an outdoor temperature at which the auxiliary furnace 324operates in response to a heat request instead of the heat pump 322.Additionally or alternatively, the user interface 302 enables a user tomanually select a thermostat control algorithm instead of the economicbalance point algorithm for control of the hybrid heating system 320.The thermostat control algorithm (e.g., implemented by thermostat 302)may, for each heating cycle, initialize a first heating stage in whichthe heat pump 322 is active without the auxiliary furnace 324 and, ifneeded, initialize a second heating stage in which the auxiliary furnace324 is active without the heat pump 322.

FIGS. 4A-4J show windows of a user interface program for controllinghybrid heating in accordance with an embodiment of the disclosure. Theuser interface program may be part of the user interface 302 describedfor FIG. 3. In FIG. 4A, window 400A shows a “settings” menu including adual fuel icon 402 that can be selected by clicking on it. Selection ofthe dual fuel icon 402 enables a user to adjust control features for ahybrid heating system (e.g., the hybrid heating system 320 of FIG. 3).The other icons of FIG. 4A correspond to other control features orutilities accessible via the user interface program.

In FIG. 4B, window 400B shows a dual fuel menu that appears in responseto clicking the dual fuel icon 402 of FIG. 4A. The dual fuel menu ofwindow 400B enables a user to manually adjust control features and/orcontrol parameter values for a hybrid heating system. For example,clicking on the comfort box 408 and then clicking the “next” button 410Aenables a user to pass control of the hybrid heating system to athermostat (e.g., thermostat 202 of FIG. 2). When the thermostatcontrols the hybrid heating system, use of the economic balance pointalgorithm is temporarily disabled or is otherwise ignored. Thethermostat may implement a thermostat control algorithm that, for eachheating cycle, initializes a first heating stage in which the heat pump322 is active without the auxiliary furnace 324. If needed (e.g., whenthe heat pump 322 is insufficient), the thermostat control algorithminitializes a second heating stage in which the auxiliary furnace isactive without the heat pump.

Clicking on the operating cost box 404 and then clicking on the “next”button 410A enables a user to input values for control parameters (e.g.,control parameters 314 of FIG. 3) of an economic balance pointalgorithm. In other words, selection of the operating cost box 404causes implementation of the economic cost balance algorithm for thehybrid heating system 320. The control parameter values for the economicbalance point algorithm are input by a user via the user interfaceprogram as shown in the windows of FIGS. 4C-4F. Additionally oralternatively, one or more default values may be provided in the userinterface program for the economic balance point algorithm as shown inthe windows of FIGS. 4G-4H.

FIGS. 4C-4H show various windows that enable selection of controlparameter values for an economic balance point algorithm. The windows ofFIGS. 4C-4H may be displayed in series, for example, after clicking onthe operating cost box 404 and the “next” box 410A. In FIG. 4C, thewindow 400C enables a user to select a gas furnace box 412 or an oilfurnace box 414. In other words, the economic balance point algorithmaccounts for the type of fuel used with the auxiliary furnace 324. Uponclicking the gas furnace box 412 and the “next” button 410B, the window400E of FIG. 4E is displayed by the user interface program.Alternatively, upon clicking the oil furnace box 414 and the “next”button 410B, the window 400F is displayed by the user interface program.In window 400C, selection of the “back” button 416A causes the dual fuelmenu window 400B to be displayed again.

In FIG. 4D, a window 400D with an electricity cost utility 418 is shown.The electricity cost utility 418 enables a user to enter an electricitycost (dollars/kwh). After entering the electricity cost, a user selectsthe “next” button 410C to use the entered electricity cost with theeconomic balance point algorithm. More specifically, the electricitycost is used to determine a cost of operating the heat pump 322. Inscreenshot 400D, selection of the “back” button 416B causes window 400Cto be displayed again.

In FIG. 4E, a window 400E with a gas cost utility 422 is shown. Thewindow 400E is displayed if the gas furnace box 412 is selected inwindow 400C. The gas cost utility 422 enables a user to enter a naturalgas cost in dollars/therm by clicking the “natural gas $/therm” button420. Alternatively, the gas cost utility 422 enables a user to enter anatural gas cost in dollars/MCF by clicking the “natural gas $/MFC”button 424. Alternatively, the gas cost utility 422 enables a user toenter a propane gas cost in dollars/gallon by clicking the “propane gas$/gal” button 426. After entering a gas cost, a user selects the “next”button 410D to use the entered gas cost with the economic balance pointalgorithm. More specifically, the gas cost is used to determine a costof operating the auxiliary furnace 324. In window 400E, selection of the“back” button 416C causes window 400D to be displayed again.

In FIG. 4F, a window 400F with an oil cost utility 430 is shown. Thewindow 400F is displayed if the oil furnace box 414 is selected inwindow 400C. The oil cost utility 430 enables a user to enter a fuel oilcost in dollars/gallon by clicking the “fuel oil $/gal” button 428.Alternatively, the “fuel oil $/gal” button 428 need not be clicked sinceonly one fuel oil cost option is provided. After entering a fuel oilcost, a user selects the “next” button 410E to use the entered fuel oilcost with the economic balance point algorithm. More specifically, thefuel oil cost is used to determine a cost of operating the auxiliaryfurnace 324. In window 400F, selection of the “back” button 416D causesscreenshot 400D to be displayed again.

In FIG. 4G, a window 400G with an Annual Fuel Utilization Efficiency(AFUE) rating utility 432 is shown. The AFUE rating utility 432 enablesa user to adjust an AFUE rating corresponding to the auxiliary furnace324. The AFUE rating for the AFUE rating utility 432 may be initiallyset to a default value (e.g., 78) and may be adjusted within apredetermined range (e.g., 78-98). After entering an AFUE rating, a userselects the “next” button 410F to use the entered AFUE rating with theeconomic balance point algorithm. More specifically, the AFUE rating isused to determine a cost of operating the auxiliary furnace 324. Inwindow 400G, selection of the “back” button 416E causes either window400E or window 400F to be displayed again.

In FIG. 4H, a window 400H with a Heating Season Performance Factor(HSPF) rating utility 434 is shown. The HSPF rating utility 434 enablesa user to adjust an HSPF rating corresponding to the heat pump 322. TheHSPF rating for the HSPF rating utility 434 may be initially set to adefault value (e.g., 7.7) and may be adjusted within a predeterminedrange (e.g., 7.7-12). After entering an HSPF rating, a user selects the“next” button 410G to use the entered HSPF rating with the economicbalance point algorithm. More specifically, the HSPF rating is used todetermine a cost of operating the heat pump 322. In window 400H,selection of the “back” button 416F causes the window 400G to bedisplayed again.

In FIG. 4I, a window 400I with a determined outdoor temperature balancepoint utility 436 is shown. The determined outdoor temperature balancepoint utility 436 shows results of an outdoor temperature balance pointdetermined by the economic balance point algorithm (referred to as the“furnace heating outdoor temperature” in utility 436) based on controlparameter values entered via the user interface program (e.g., via theutilities of window 400D-400H). The determined outdoor balance pointutility 436 also enables a user to adjust the determined outdoortemperature balance point up or down. To accept the determined outdoortemperature balance point or an adjusted outdoor temperature balancepoint, the user selects the “accept” button 438A. The user mayalternatively click the “cancel” button 440A to cancel use of thedetermined outdoor temperature balance point or adjusted outdoortemperature balance point to control a hybrid heating system 320.

Returning to FIG. 4B, clicking on the outdoor temperature box 406 andthen clicking on the “next” button 410A enables a user to manually setan outdoor temperature balance point. When the outdoor temperature is ator above the outdoor temperature balance point, the heat pump 322 isselected in response to a heat request. When the outdoor temperature isbelow the outdoor temperature balance point, the auxiliary furnace 324is selected in response to a heat request. FIG. 4J shows a window 400Jwith a custom outdoor temperature balance point utility 437. The customoutdoor temperature balance point utility 437 enables a user to select acustom outdoor temperature balance point (referred to as the “furnaceheating outdoor temperature” in utility 437) between 0-70 degreesFahrenheit. Other temperature ranges could or selection means couldalternatively be used. Once a custom outdoor temperature balance pointis selected in utility 437, a user clicks the “accept” button 438B toimplement use of the custom outdoor temperature balance point. The usermay alternatively click the “cancel” button 440B to cancel use of acustom temperature balance point.

Although windows 400C-400J describe various features and utilities in aparticular order, the windows presented herein are not intended to limitother user interface embodiments that may implement an economic balancepoint algorithm as described herein. In other words, user interfaceembodiments may vary with regard to how information is presented to auser and how a user enters information.

FIG. 5 shows a method 500 in accordance with an embodiment of thedisclosure. The method 500 may be performed by a controller (e.g.,controller 310) or control system for hybrid fuel heating of an HVACsystem as described herein. As shown, the method 500 comprisesdetermining an outdoor temperature balance point at which operating anauxiliary furnace is less expensive that operating a heat pump (block502). The determined outdoor temperature balance point may be based oncontrol parameters such as an auxiliary furnace fuel cost parameter, aheat pump electricity cost parameter, a heat pump efficiency parameterand an auxiliary furnace efficiency parameter. At block 504, a heatrequest is received. Finally, an auxiliary furnace or heat pump isselected (responsive to the heat request) based on the determinedoutdoor temperature balance point (block 506).

In at least some embodiments, the method 500 may enable determination ofthe outdoor temperature balance point to be disabled or overridden by auser. For example, a user may enter a custom outdoor temperature balancepoint. Further, a user may select to implement a thermostat controlscheme instead of an economic balance point algorithm. The thermostatcontrol scheme comprises, for example, initializing a first heatingstage in which the heat pump is active without the auxiliary furnace. Ifneeded, thermostat control scheme initializes a second heating stage inwhich the auxiliary furnace is active without the heat pump.

Preferred embodiments have been described herein in sufficient detail,it is believed, to enable one skilled in the art to practice thedisclosed embodiments. Although preferred embodiments have beendescribed in detail, those skilled in the art will also recognize thatvarious substitutions and modifications may be made without departingfrom the scope and spirit of the appended claims.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations should be understood to include iterative rangesor limitations of like magnitude falling within the expressly statedranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4,etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, Rl, and an upper limit,Ru, is disclosed, any number falling within the range is specificallydisclosed. In particular, the following numbers within the range arespecifically disclosed: R=Rl+k*(Ru−Rl), wherein k is a variable rangingfrom 1 percent to 100 percent with a 1 percent increment, i.e., k is 1percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . 50 percent,51 percent, 52 percent, . . . , 95 percent, 96 percent, 97 percent, 98percent, 99 percent, or 100 percent. Moreover, any numerical rangedefined by two R numbers as defined in the above is also specificallydisclosed. Use of the term “optionally” with respect to any element of aclaim means that the element is required, or alternatively, the elementis not required, both alternatives being within the scope of the claim.Use of broader terms such as comprises, includes, and having should beunderstood to provide support for narrower terms such as consisting of,consisting essentially of, and comprised substantially of. Accordingly,the scope of protection is not limited by the description set out abovebut is defined by the claims that follow, that scope including allequivalents of the subject matter of the claims. Each and every claim isincorporated as further disclosure into the specification and the claimsare embodiment(s) of the present invention.

1. A hybrid heating system, comprising: a heat pump; an auxiliaryfurnace; and a controller coupled to the heat pump and the auxiliaryfurnace, wherein the controller, in response to receiving a heatrequest, selects either the heat pump or the auxiliary furnace based onan economic balance point algorithm.
 2. The hybrid heating system ofclaim 1 wherein the economic balance point algorithm comprises anauxiliary furnace fuel cost parameter, a heat pump electricity costparameter, a heat pump efficiency parameter, and an auxiliary furnaceefficiency parameter.
 3. The hybrid heating system of claim 2 whereinthe economic balance point algorithm implements default values for atleast one of the auxiliary furnace fuel cost parameter, the heat pumpelectricity cost parameter, the heat pump efficiency parameter, and theauxiliary furnace efficiency parameter.
 4. The hybrid heating system ofclaim 2 wherein the controller comprises a user interface and whereinvalues for at least one of the auxiliary furnace fuel cost parameter,the heat pump electricity cost parameter, the heat pump efficiencyparameter, and the auxiliary furnace efficiency parameter are based onuser input via the user interface.
 5. The hybrid heating system of claim1 wherein the economic balance point algorithm determines an outdoortemperature balance point at which operating the auxiliary furnace isless expensive than operating the heat pump.
 6. The hybrid heatingsystem of claim 5 wherein the controller, in response to receiving aheat request, compares a current outdoor temperature with a previouslydetermined outdoor temperature balance point and selects either the heatpump or the auxiliary furnace based on the comparison.
 7. The hybridheating system of claim 1 wherein the controller comprises a userinterface that enables a user to selectively disable and enable theeconomic balance point algorithm.
 8. The hybrid heating system of claim1 wherein the controller couples to a user interface that enables a userto manually set an outdoor temperature at which the auxiliary furnaceoperates in response to a heat request instead of the heat pump.
 9. Thehybrid heating system of claim 1 wherein the controller selectivelyimplements a thermostat control algorithm instead of the economicbalance point algorithm based on user input.
 10. The hybrid heatingsystem of claim 9 wherein the thermostat control algorithm, for eachheating cycle, initializes a first heating stage in which the heat pumpis active without the auxiliary furnace and, if needed, initializes asecond heating stage in which the auxiliary furnace is active withoutthe heat pump.
 11. A control system for a hybrid heating system, thecontrol system comprising: economic balance point logic configured todetermine an outdoor temperature threshold at which operating anauxiliary furnace is less expensive than operating a heat pump;selection logic configured to select, in response to a heat request,either the auxiliary furnace or the heat pump based on the outdoortemperature threshold.
 12. The control system of claim 11 wherein theeconomic balance point logic determines the output temperature thresholdbased on an auxiliary furnace fuel cost parameter, a heat pumpelectricity cost parameter, a heat pump efficiency parameter, and anauxiliary furnace efficiency parameter.
 13. The control system of claim12 wherein the economic balance point logic implements default valuesfor at least one of the auxiliary furnace fuel cost parameter, the heatpump electricity cost parameter, the heat pump efficiency parameter, andthe auxiliary furnace efficiency parameter.
 14. The control system ofclaim 11 further comprising a user interface in communication with theeconomic balance point logic, wherein values for at least one of theauxiliary furnace fuel cost parameter, the heat pump electricity costparameter, the heat pump efficiency parameter, and the auxiliary furnaceefficiency parameter are based on user input via the user interface. 15.The control system of claim 12 further comprising a user interface incommunication with the selection logic, wherein the selection logic isconfigured to select either the auxiliary furnace or the heat pump for aheat cycle based on an outdoor temperature value or a thermostat controlscheme selected manually by a user via the user interface.
 16. A methodfor controlling a hybrid heating system, comprising: determining, by acontroller, an outdoor temperature threshold at which operating anauxiliary furnace is less expensive than operating a heat pump;receiving, by the controller, a heat request; and selecting, by thecontroller, either the auxiliary furnace or the heat pump based on thedetermined outdoor temperature threshold.
 17. The method of claim 16wherein said determining the outdoor temperature threshold is based onan auxiliary furnace fuel cost parameter, a heat pump electricity costparameter, a heat pump efficiency parameter, and an auxiliary furnaceefficiency parameter.
 18. The method of claim 16 further comprisingoverriding the determined outdoor temperature threshold with an outdoortemperature provided by a user.
 19. The method of claim 16 furthercomprising disabling use of the determined outdoor temperature thresholdfor said selection and enabling use of a thermostat control scheme toselect either the auxiliary furnace or the heat pump based on thedetermined outdoor temperature threshold.
 20. The method of claim 19wherein the thermostat control scheme comprises initializing a firstheating stage in which the heat pump is active without the auxiliaryfurnace and, if needed, initializing a second heating stage in which theauxiliary furnace is active without the heat pump.